The present invention relates to the making of iron powder particularly of the variety which has fibrous configuration, structure and texture.
The art and technique of making sintered parts requires usually iron powder with a bulk density from 2.3 to 3.5 grams per cubic centimeter. Aside therefrom, powder having a much lower bulk density is of interest in other fields, such as the manufacture of friction and brake linings. Specifically, such linings have a metal, skeleton-like body with many pores for embedding adequate quantities of another material, e.g. graphite or plastic. The skeleton is made of medium light weight powder having bulk density of about 1.4 to 1.5 g/cm.sup.3.
Such medium light weight iron powder is made in accordance with a known method as follows. Very pure magnetic iron ore concentrate is reduced to sponge iron in a muffle or retort furnace at a temperature of 700.degree.C whereby the furnace is additionally charged with coke fines and a solid desulfurizing agent. Iron sponge from the residue is ground to iron powder. This method is, for example, described in "Etude bibliographique des procedes de reduction direct des minerais de fer", 3rd edition, B/2, Communaute europeenne du charbon et de 1'acier haute autorite, Luxembourg May 1967.
The "Journal Iron and Steel Inst.", London, 1956 pages 90 to 96, describes a method of making still lighter iron powder. Iron oxide (Fe.sub.2 O.sub.3) is reduced to sponge iron in a pure hydrogen atmosphere at temperatures from 760.degree. to 980.degree.C. The iron sponge is then broken into particles by means of wire brushes to obtain a powder having grain size from 53 to 74 .mu.. Subsequently, the powder is heated in hydrogen to a temperature within the narrow range from 704.degree. to 717.degree. C in order to reduce any residual oxide. The bulk density of that powder is reported to be 0.79 g/cm.sup.3.
Investigations and tests have demonstrated that powder particles made by this method are themselves spongy and very porous. It was found further that such an iron powder is completely unusable in any economical process, for example, because it has a very low fluidity.
Light weight iron powder made by the known reduction processes have these disadvantageous properties because of specific inherent relations that underlie the reduction of iron oxides and will be explained next. As the iron oxide is being reduced, the oxygen as produced and extracted diffuses through the still solid ore particles. The loss in mass causes formation of pores in these particles. The total volume of the pores thus produced can be calculated from the difference in density between the raw product (hematite) and the iron that results from the process. Accordingly, it was found that 1 cm.sup.3 raw material (nonporous hematite particles) changed in that 53 % of that volume was occupied by pores. In other words, the particles which did not change in shape, were now constituted by 0.47 cm.sup.3 metallic iron, the remainder volume was occupied by pores. The mechanism of extracting oxygen differs with the method and depends upon the ore type as well as on the conditions under which reduction is carried out (e.g. reduction temperature, composition of the reduction medium). Accordingly, the morphological structure of the iron as made may differ so that number, shape, size, and distribution of the pores in the reduced iron may vary greatly. Differences in morphological structure of the reduced iron particles are reflected in different properties of the powder as made; see, for example, "Bockstiegel, G., Int. Journ. of Powder Metallurgy 2 (4) 1966".
For example, if the oxide is reduced at a low temperature (e.g. 700.degree. C) a powder is produced having, indeed, a low bulk density. However, the powder particles as made in that manner, have a very high micro-porosity which may even result in pyrophoric properties. In other words, the low bulk density is to some extent fictitious because it is not only the result of external (usual) voids between particles but also of micro-porosity of the particles themselves. Even those pores which are theoretically open are actually accessible only with difficulties due to labyrinth-like connections. Thus, these particular pores impede significantly the compacting of the powder material by a press. Moreover, these more or less closed pores are not available or to a small extent only to receive other substances for obtaining a compound material.
Another disadvantage of using low reduction temperatures is to be seen in that inherently long periods of times are needed to complete the process; quite frequently the long process time is coupled with a high consumption of reducing agents. Still further disadvantages are to be seen in that in some cases one produces iron particles, which are rather weak mechanically, so that the powder must be treated very gently. As was already mentioned above, known powder having low bulk density is not very fluid. However, all these disadvantages have been allowed for, if such a material had to be used for reasons of its desirable properties.
Iron powder having no or very few pores and, therefore, being free from the deficiencies outlined above have been made in the past only by atomizing molten iron by means of a pressurized fluid. The resulting, rather compact iron particles pack very tightly, and the powder has a high bulk density accordingly, i.e. in excess of 2.3 g/cm.sup.3, see, for example, in Michalke, M. and W. Scholz "Die Erzeugung von Eisen- und Legierungspulvern durch Zerstaeuben von Schmelzen", 2nd Europ. Symposium on powder metallurgy, Stuttgart, May 8 to 10, 1968.